Polymer drug delivery systems have been developed for the controlled release of pharmaceutical polypeptides. For example, synthetic polyesters such as poly(DL-lactic acid), poly(glycolic acid), poly(lactic-glycolic acid), and poly(.epsilon.-caprolactone) have been used in the form of microcapsules, films, or rods to release biologically active polypeptides. See e.g., U.S. Pat. Nos. 4,767,628 and 4,675,189 and PCT Application No. WO 94/00148.
In addition to the synthetic polymeric chains, natural polymers and their derivatives have been used as components in similar sustained release compositions that dissociate by enzymatic degradation. One example of such natural polymers are those based on chitin, a poly(N-acetylglucosamine). However, since chitin is water insoluble, others have examined solubilizable derivatives which are based primarily on a partially deacetylated chitin, e.g., chitosan. See e.g., Sanford, P. A. et al., Eds., Advances in Chitin & Chitosan (1992). Although chitosan can be found in some fungi, the production of biodegradable chitosan is generally performed synthetically. See Mima, et. al., J. Appl. Polym. Sci. 28:1909-1917 (1983). Synthetic derivatives of chitosan have also been prepared to alter the polymer's in vivo biological characteristics. See Muzzarelli, et al., Carbohydrate Res. 207:199-214 (1980).
The use of chitin, as well as chitin derivatives, has been proposed in a number of drug delivery systems. See, e.g., European Patent Application Nos. 486,959, 482,649, 525,813 A1, and 544,000 A1; and U.S. Pat. No. 5,271,945.